Barack Obama and Vladimir Putin on Monday aired clashing views on Syria, but pledged to at least try to keep alive a frail and much-delayed effort to hold a Geneva peace conference.

The US and Russian presidents faced off on the sidelines of the G8 summit in Northern Ireland, days after the White House signalled it would begin arming rebels battling President Bashar al-Assad, Moscow's top Arab ally.

Putin and Obama did not try to disguise the fact they are estranged on Syria and cannot agree on Assad's fate -- but appeared keen to keep flagrant differences on the vicious civil war from sinking wider US-Russia relations.

They announced Obama would go to Moscow on September 3-4 for a full-scale summit, expanding a previously announced trip that includes the G20 summit in St Petersburg.

They spoke of cooperation on terrorism and economics, with Russia now in the World Trade Organization, and agreed to revive a frozen agreement which secures loose nuclear and radioactive materials in the former Soviet Union.

But on Syria, the differences remain deep.

"Of course our opinions do not converge, but all of us have the intention to stop the violence in Syria and stop the growth in the number of victims," said Putin, who has scoffed at US plans to begin arming selected rebels.

He said he and Obama agreed that the vicious civil war that has killed at least 90,000 people must end "peacefully" and through talks.

"We agreed to push the parties to the negotiating table."

Obama acknowledged that they had "different perspectives" -- a formulation that vastly understates mistrust between Moscow and Washington on the war.

In a photo-op after the talks there was no sign of the chumminess that characterised meetings between Obama and former president Dmitry Medvedev, with whom he engineered a "reset" of US-Russia relations in his first term.

But the session ended with a firm handshake, after Obama made a gentle joke about the Russian leader's prowess in judo and his own declining basketball skills.

Putin had earlier faced a volley of criticism from other world leaders at the G8 summit over his position on Syria.

"It is fairly evident that there is a difference of view on Syria between the former G7 and Russia," said Ben Rhodes, a deputy US national security advisor.

Washington and Moscow have agreed in principle to host a conference in Geneva on ending the violence in Syria, but the timetable for the meeting has continually slipped over disagreements on who should attend.

Still, Obama said he and Putin would instruct their teams to keep at it.

"We share an interest in reducing the violence, securing chemical weapons and ensuring that they are neither used nor are they subject to proliferation," Obama said at the luxury Lough Erne golf resort near Enniskillen.

"We want to try to resolve the issue through political means if possible, so we will instruct our teams to continue to work on the potential of a Geneva follow-up."

Rhodes said that a full blown conference remained the goal of the talks.

Moscow and Washington are key players in the currently stalled P5+1 talks between Tehran and world powers designed to get Iran to stand down its nuclear programme, and both said they hoped Iran's election would unfreeze the process.

Obama said he and Putin had expressed "cautious optimism" that the election of the next Iranian president Hassan Rowhani, who is seen as a comparative moderate, could change the dynamics of the effort.

Putin said he hoped that the vote would reveal "new opportunities to solve the Iranian nuclear problem."

Obama said that after the bombing of the Boston Marathon this year, blamed on two men hailing from the Russian Caucuses region, both sides had an interest in counter-terrorism cooperation.